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Hibiscus needs full sun; however, most experts agree that the ideal sun conditions for hibiscus include 2-4 hours of direct sunlight with the rest of the time in partial shade.

There are several reasons this could be happening. This article can help you learn which one it is: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm

It might be possible, but it could be difficult with tropical hibiscus. If you get many freezing nights, even with the extra protection, they would have trouble surviving. Tropical hibiscus, if it survived, would need the extra protection every year. They may have been talking about hardy hibiscus as well, and that needs no extra winter protection where you live.

The proper time to transplant the hibiscus is anywhere in late fall or early spring, depending on how cold your climate is and the variety you're growing. Hibiscus enjoy warm, sunny conditions, especially the tropical variety (which should remain in these conditions). However, hardier varieties can withstand cooler temps.

I would check the roots. It may have root rot. This would prevent it from taking up water even after it has been watered. This article will help with that:
https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm

Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-flowers-hibiscus-blossoms-falling-off-plant.htm